[tbob22]

Core 2 Duos and Pentium Dual Cores under 3ghz are fairly energy efficient chips (~65w) and they use Speedstep, etc to keep power consumption low at idle.

Of course newer dual cores and more modern chipsets will use less power overall, although for less demanding uses (web, word processing) I'm not sure if there would be a huge difference.

P4's on the other hand...